1. Field of the Invention
The present invention relates to a single mode optical fiber (hereinafter referred to as S-mode optical fiber) used for transmitting light in long-haul optical communications or the like and, in particular, to a dispersion-shifted fiber suitable for wavelength-multiplexing transmission.
2. Related Background Art
Conventionally, optical communication systems employing a S-mode optical fiber as their transmission line have often utilized light in the wavelength hand of 1.3 .mu.m or 1.55 .mu.m as their signal light for communications. Recently, in order to reduce transmission loss in the transmission line, the light in the wavelength band of 1.55 .mu.m has been in use more and more. The S-mode optical fiber employed in such a transmission line for light in the wavelength band of 1.55 .mu.m (hereinafter referred to as 1.55-.mu.m S-mode optical fiber) has been designed such that its wavelength dispersion (phenomenon in which pulse wave spreads due to the fact that velocity of propagation of light changes depending on its wavelength) is nullified (namely, to yield a dispersion-shifted fiber whose zero-dispersion wavelength is 1.55 .mu.m). For example, as such a dispersion shifted fiber, Japanese Patent Publication No. 3-18161 discloses a dispersion-shifted fiber having a dual-shape core type refractive index profile in which a core is constituted by an inner core layer and an outer core layer having a refractive index lower than that of the inner core layer. Further, Japanese Patent Application Laid-Open No. 63-43107 and No. 2-141704 propose a dispersion-shifted fiber having a depressed cladding/dual-shape core type refractive index profile in which, in addition to the double core structure mentioned above, a cladding is constituted by an inner cladding layer and an outer cladding layer having a refractive index higher than that of the inner cladding layer.
On the other hand, long-haul light transmission has recently become possible with the advent of wavelength division multiplex (WDM) transmission and optical amplifiers. Under such circumstances, however, influences of nonlinear optical effects cannot be neglected. Accordingly, in order to eliminate the nonlinear optical effects, it has been proposed to deform the refractive index profiles mentioned above, thereby shifting their zero-dispersion wavelength toward the shorter or longer wavelength side of their signal wavelength band (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, a nonlinear optical effect is a phenomenon in which a signal light pulse is distorted in proportion to density of light intensity or the like. This phenomenon becomes a factor restricting transmission speed, as well as a relay distance in a relaying transmission system.